1. Field of Invention
The present invention relates to unstable resonators and more particularly to unstable laser resonators having an optical path which extends in a radial direction.
2. Description of the Prior Art
In various unstable laser resonators such as those typified by U.S. Pat. No. 3,921,096, entitled Unstable Split Mode Resonator, filed on Dec. 16, 1974, U.S. Pat. No. 3,950,712, entitled Unstable Laser Resonator Having Radial Propagation, filed Apr. 14, 1975, U.S. Pat. No. 3,969,687, entitled Standing Wave Unstable Resonators For Radial Flow Lasers, filed Apr. 14, 1975, and U.S. Pat. No. 3,969,688, entitled Traveling Wave Unstable Resonators For Radial Flow Lasers, filed Apr. 10, 1975, each of which is held with the present invention by common assignee, there is a need to transform the circulating power from an annular or a radially extending region into a circular beam from which power is coupled out of the resonator. Such transformations are typically accomplished with a folding mirror which, for example, is a cone having a conical reflective surface which terminates at a point apex or a triangular prism having two flat reflective surfaces which intersect along a straight line apex. The presence of a folding mirror having a sharp discontinuity due to an apex causes the resonator to operate in other than its lowest order mode which in turn reduces the amount of energy on axis in the far field energy distribution for the beam of radiation coupled out of such a resonator. Any techniques which would allow an unstable resonator having a mirror with a severe discontinuity such as results at the apex of a conical folding mirror to resonate at the lowest order symmetric mode possible would broaden the fields of application for unstable laser resonators.